Cardboard Box Car - Both girl's and boys love cars. It's probably because they see you, the people they learn everything from, driving in one all the time. I've suggested making this to some friends that have little girls and they said "oh that's for boys". I beg to differ, the girls will enjoy it just as much. Once they see the finished product, that's when their imagination will take over. All you got to do is build it! This is one of many designs around the net. Add your own flair, acquire a bigger box and make a bus!
Dry Ice Bubble (or just fun with dry ice) - I remember this being one of my favorite things when I was a little one. Every time my parents would go to the grocery store with or without us we would always ask if they would get some dry ice. I was into medieval stuff when I was young so it served as some cursed fog around a castle or something similar to that to play in. I would play with it until the last poof of fog came out. Try doing the bubble thing when it first starts, and then let their imaginations do the rest. This stuff is pretty cheap, so if you have a really big bowl, go all out. :::WARNING: Your children should not touch the dry ice or breath in the fog so this is not good for kids who are not yet old enough to listen and understand that it can hurt them!:::
Art - Now obviously this is a broad activity subject. There are many many different ways to do art with your kids, most of which make a mess. I guess that comes with the territory though, right? Art can be a very good independent activity for your kids when you are trying to get some stuff done (emphasis on trying). So some things you can do: start with this simple idea. Now as a parent you might be thinking, well how long is that going to last? So if and when they get bored of the markers, bust out the paint and "tools". Before you clean up the mess, maybe finish up the art time with something a little more calm and organized than free wheelin it, because it will be naptime right afterwards right? Correct. :) N